Pharmacokinetics of remifentanil in conscious cats and cats anesthetized with isoflurane

Bruno H. Pypendop Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616.

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Robert J. Brosnan Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616.

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Kristine T. Siao Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616.

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Scott D. Stanley K. L. Maddy Equine Analytical Chemistry Laboratory, School of Veterinary Medicine, University of California, Davis, CA 95616.

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Abstract

Objective—To characterize the pharmacokinetics of remifentanil in conscious cats and cats anesthetized with isoflurane.

Animals—6 cats.

Procedures—Remifentanil (1 μg/kg/min for 5 minutes) was administered IV in conscious cats or cats anesthetized with 1.63% isoflurane in oxygen in a randomized crossover design. Blood samples were obtained immediately prior to remifentanil administration and every minute for 10 minutes, every 2 minutes for 10 minutes, and every 5 minutes for 10 minutes after the beginning of the infusion. Blood was immediately transferred to tubes containing citric acid, flash frozen in liquid nitrogen, and stored at −80°C until analysis. Blood remifentanil concentration was determined by use of liquid chromatography–mass spectrometry. Remifentanil concentration-time data were fitted to compartment models.

Results—A 2-compartment model (with zero-order input because of study design) best described the disposition of remifentanil in awake and isoflurane-anesthetized cats. The apparent volume of distribution of the central compartment, the apparent volume of distribution at steady state, the clearance, and the terminal half-life (median [range]) were 1,596 (1,164 to 2,111) and 567 (278 to 641) mL/kg, 7,632 (2,284 to 76,039) and 1,651 (446 to 29,229) mL/kg, 766 (408 to 1,473) and 371 (197 to 472) mL/min/kg, and 17.4 (5.5 to 920.3) and 15.7 (3.8 to 410.3) minutes in conscious and anesthetized cats, respectively.

Conclusions and Clinical Relevance—The disposition of remifentanil in cats was characterized by a high clearance. Isoflurane anesthesia significantly decreased the volume of the central compartment, likely by decreasing blood flow to vessel-rich organs.

Abstract

Objective—To characterize the pharmacokinetics of remifentanil in conscious cats and cats anesthetized with isoflurane.

Animals—6 cats.

Procedures—Remifentanil (1 μg/kg/min for 5 minutes) was administered IV in conscious cats or cats anesthetized with 1.63% isoflurane in oxygen in a randomized crossover design. Blood samples were obtained immediately prior to remifentanil administration and every minute for 10 minutes, every 2 minutes for 10 minutes, and every 5 minutes for 10 minutes after the beginning of the infusion. Blood was immediately transferred to tubes containing citric acid, flash frozen in liquid nitrogen, and stored at −80°C until analysis. Blood remifentanil concentration was determined by use of liquid chromatography–mass spectrometry. Remifentanil concentration-time data were fitted to compartment models.

Results—A 2-compartment model (with zero-order input because of study design) best described the disposition of remifentanil in awake and isoflurane-anesthetized cats. The apparent volume of distribution of the central compartment, the apparent volume of distribution at steady state, the clearance, and the terminal half-life (median [range]) were 1,596 (1,164 to 2,111) and 567 (278 to 641) mL/kg, 7,632 (2,284 to 76,039) and 1,651 (446 to 29,229) mL/kg, 766 (408 to 1,473) and 371 (197 to 472) mL/min/kg, and 17.4 (5.5 to 920.3) and 15.7 (3.8 to 410.3) minutes in conscious and anesthetized cats, respectively.

Conclusions and Clinical Relevance—The disposition of remifentanil in cats was characterized by a high clearance. Isoflurane anesthesia significantly decreased the volume of the central compartment, likely by decreasing blood flow to vessel-rich organs.

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